1. Field of the Invention
The present invention relates to a hydraulic control system for a transmission in which a driving rotary member, a driven rotary member and a transmission member are controlled in their contact pressures by a higher oil pressure and in which frictional engagement elements are controlled in their applications/releases by a lower oil pressure.
2. Related Art
Generally, a discontinuous transmission capable of controlling gear ratios stepwise (or discontinuously) and automatically is provided with a gear speed change mechanism having a plurality of planetary gear mechanisms, a plurality of frictional engagement elements for changing the torque transmission lines of the gear speed change mechanism and a hydraulic control system for controlling the applications/releases of those frictional engagement elements with oil pressure. In this discontinuous transmission, the frictional engagement elements are controlled in their applications/releases to change the gear ratios by controlling the oil pressures acting on the frictional engagement elements on the basis of predetermined conditions. Since output pressure of an oil pressure source of the hydraulic control system is set higher than a line pressure or an initial pressure of the entire hydraulic circuit, there arises a problem of the so-called xe2x80x9cshift shockxe2x80x9d when the output pressure of the oil pressure source is used as it is as the pressure for applying the frictional engagement elements. In Japanese Patent Laid-Open No. 8-285067 (JP-A-8-285067), therefore, there is disclosed an example of a hydraulic control system for an automatic transmission, in which the output pressure of the oil pressure source is not used as the pressure for applying the frictional engagement elements before it is lowered to a predetermined level.
The automatic transmission, as disclosed in the Laid-Open, is arranged on the output side of an engine and is provided with a torque converter, a gear speed change mechanism having a plurality of planetary gear mechanisms, a plurality of frictional engagement elements and a hydraulic control system. This hydraulic control system is provided with: an oil pump as the oil pressure source; a primary regulator valve for regulating the output pressure of the oil pump to a line pressure according to the vehicle speed and the throttle opening; a manual valve for receiving the line pressure to output an oil pressure in a manner to correspond to each shift position; and a modulator valve for lowering the oil pressure, as outputted from the manual valve, in a shift position requiring an engine braking force, to a pressure to be fed to the hydraulic servos of the frictional engagement elements. When the shift position requiring the engine braking force is selected, moreover, the shift shock at the engine braking can be suppressed by feeding the frictional engagement elements with the oil pressure which is made lower than the oil pressure to be fed to the frictional engagement element to be applied when a predetermined gear stage is set.
Here, the transmission capable of controlling the gear ratios automatically is exemplified not only by the discontinuous transmission disclosed in the Laid-Open but also by a continuously variable transmission capable of controlling the gear ratios without any stage (i.e., continuously). As an example of this continuously variable transmission, there can be enumerated the belt-type continuously variable transmission which is provided with a driving pulley (primary pulley), a driven pulley (secondary pulley) and a belt. In the outer circumferences of the driving pulley and the driven pulley, there are individually formed V-shaped grooves, in which the belt is wound. On the other hand, a hydraulic control system is provided for controlling the widths of the grooves of the driving pulley and the driven pulley.
By controlling the groove width of the driving pulley, moreover, the winding radius of the belt (i.e., the effective radius of the pulley) is changed to control the gear ratio. By adjusting the groove width of the driven pulley, i.e., the clamping force for the belt, on the other hand, the tension of the belt is controlled. Thus, the contact pressures between the driving pulley and the driven pulley, and the belt can be controlled to retain the torque to be inputted to the continuously variable transmission and the torque transmission capacity according to the gear ratio.
In the belt type continuously variable transmission, on the other hand, the driving pulley and the driven pulley are restricted in their external diameters by the relation between the positions of the driving pulley and the driven pulley, and those of the parts to be arranged around the driving pulley and the driven pulley. These positional relations limit the enlargement in the control range of the gear ratio, as controlled only by the belt type continuously variable transmission. Thus, there has been proposed a layout in which the control width of the gear ratio is enlarged by arranging the belt type continuously variable transmission and a transmission having the planetary gear mechanism and the frictional engagement elements, as disclosed in the Laid-Open, in tandem in the torque transmission line leading from the engine to the wheels. In the vehicle adopting this layout, the oil pressure sources for outputting initial pressures of the oil pressure for controlling the groove width of the driven pulley and the oil pressure for controlling the applications/releases of the frictional engagement elements are desired to be common for suppressing the number of parts of the hydraulic control system and for reducing the size/weight of the hydraulic control system.
Here will be compared the belt type continuously variable transmission and the transmission having the gear speed change mechanism. The oil pressure for establishing a clamping pressure for the belt and the oil pressure necessary for applying the frictional engagement elements are made different by the difference in the power transmission manners. In the belt type continuously variable transmission, more specifically, the torque transmission capacity is retained by the frictional contacts between the driving pulley and the driven pulley of relatively smaller diameters and a portion of the belt. In the transmission having the gear speed change mechanism, on the contrary, the torque transmission capacity is retained by the frictional contact of the entirety of the annular frictional faces of the frictional engagement elements having relatively larger external diameters. As a result, the oil pressure necessary for controlling the groove width of the driven pulley in the belt type continuously variable transmission needs to be higher than the oil pressure necessary for applying the frictional engagement elements in the automatic transmission having the gear speed change mechanism.
When an output pressure of the oil pressure source is controlled to a high level and a high oil pressure (i.e., a high output pressure) is fed to the frictional engagement elements, however, the components of the hydraulic circuit leading to the frictional engagement elements are required to have the strength for resisting the high oil pressure, thus causing a problem that the hydraulic control system is enlarged in its size, increased in its weight and raised in its manufacture cost. Additionally, the output pressure of the oil pressure source acts as it is on the frictional engagement elements so that the frictional engagement elements have to be protected against the high oil pressure. As a result, the frictional engagement elements may also be enlarged in their size, increased in their weight and raised in their manufacture cost and may be damaged.
An object of the invention is to provide a hydraulic control system for a transmission, which can be reduced in size and weight and lowered in cost while preventing frictional engagement elements in advance from being damaged.
Another object of the invention is to reduce the size and weight and lower the cost for a hydraulic control system for a transmission including: a continuously variable transmission having a transmission torque influenced by a clamping pressure based on an oil pressure; and a frictional engagement elements adapted to be applied by the oil pressure for transmitting the torque.
The hydraulic control system according to the invention is applied to a transmission including a first transmission mechanism to be actuated by a higher oil pressure and a second transmission mechanism to be actuated by a lower oil pressure. The oil pressure, as established by an oil pressure source, is regulated by a first pressure regulating mechanism to a first oil pressure to be fed to the first transmission mechanism, and the first oil pressure is regulated by a second pressure regulating mechanism to a second oil pressure to be fed to the second transmission mechanism. Therefore, the first oil pressure having a relatively higher level acts on the sections from the oil pressure source to the individual pressure regulating mechanisms and on the section from the first pressure regulating mechanism to the first transmission mechanism, but the second oil pressure having a relatively lower level or a pressure below the second oil pressure acts on the remaining sections. Therefore, the portions to have a structure capable of resisting the high pressure may be reduced, so that the hydraulic control system can be reduced in size and weight as a whole to lower its cost.
On the other hand, the frictional engagement elements are actuated by the relatively low second oil pressure which is regulated by the second pressure regulating mechanism, so that the relatively high first oil pressure exerts no influence on the frictional engagement elements, thereby to avoid the dispersion, as might otherwise be caused by the dispersion of the first oil pressure, of the oil pressures for applying the frictional engagement elements in advance.
Moreover, the hydraulic control system of the invention can be constructed as one for a transmission provided with a continuously speed changing mechanism and hydraulic frictional engagement elements. The relatively high first oil pressure is fed to the continuously speed changing mechanism, whereas the relatively low second oil pressure is fed to the frictional engagement elements. With a large difference between the first oil pressure and the second oil pressure, however, the first oil pressure, as established in the first pressure regulating mechanism, is regulated by the second pressure regulating mechanism to the second oil pressure and is fed to the frictional engagement elements, so that no special pressure tight structure is required for the frictional engagement elements and for a device for controlling them to suppress the dispersion of the oil pressures in the frictional engagement elements.
The above and further objects and novel features of the invention will more fully appear from the following detained description when the same is read with reference to the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustrations only and are not intended as a definition of the limits of the invention.